4.02.1 Introduction

Austenitic stainless steels are widely used as struc­tural components in nuclear service in addition to being employed in many other nonnuclear engineering and technological applications. The description of these steels and their as-fabricated properties is covered in Chapter 2.09, Properties of Austenitic Steels for Nuclear Reactor Applica­tions. This chapter describes the evolution of both microstructure and macroscopic property changes that occur when these steels are subjected not only to prolonged strenuous environments but also to the punishing effects of radiation. While various nuclear environments involve mixtures of charged particles, high-energy photons and neutrons, it is the latter that usually exerts the strongest influence on the evolution of structural steels and thereby determines the lifetime and continued functionality of structural components.

To describe the response of austenitic stainless steels in all neutron environments is a challenging assignment, especially given the wide range of neutron spectra characteristic of various neutron devices. This review of neutron-induced changes in properties and dimensions of austenitic stainless steels in all spectral environments has therefore been compiled from a series of other, more focused reviews directed toward particular reactor types1-8 and then augmented with material from a recently published textbook9 and journal articles. It should be noted, however, that many of the behavioral char­acteristics of iron-based stainless steels following neutron irradiation are also observed in nickel — based alloys. Whenever appropriate, the similarities between the two face-centered-cubic alloy systems will be highlighted. A more comprehensive treat­ment of radiation effects in nickel-base alloys is provided in Chapter 4.04, Radiation Effects in Nickel-Based Alloys.

This review is confined to the effects of neutron exposure only on the response ofirradiated steels and does not address the influence of charged particle irradiation. While most of the phenomena induced by neutrons and charged particles are identical, there are additional processes occurring in charged par­ticle studies that can strongly influence the results. Examples of processes characteristic of charged par­ticle simulations are the injected interstitial effect,10, strong surface effects,12,13 dose gradients,14,15 and atypical stress states.16,17 Chapter 1.07, Radiation Damage Using Ion Beams addresses the use of charged particles for irradiation.

Austenitic stainless steels used as fuel cladding or structural components in various reactor types must often withstand an exceptionally strenuous and chal­lenging environment, even in the absence of neutron irradiation. Depending on the particular reactor type, the inlet temperature during reactor operation can range from ^50 to ~-370 °C. The maximum temper­ature can range from as high as 650 to 700 °C for structural components in some reactor types, although most nonfueled stainless steel components reach maximum temperatures in the range of 400-550 °C. During operation, the steel must also withstand the corrosive action of fission products on some surfaces and flowing coolant on other surfaces. The coolant especially may be corrosive to the steel under operating conditions. Some of these environmental phenomena are synergized or enhanced by the effect of neutron irradiation.

Dependent on the nature of the component and the length of its exposure, there may also be sig­nificant levels of stress acting on the component. Stress not only influences cracking and corrosion (see Chapter 5.08, Irradiation Assisted Stress Cor­rosion Cracking) but can also impact the dimen­sional stability of stainless steel, primarily due to thermal creep and irradiation creep, and also from the influence of stress on precipitation, phase stabil­ity, and void growth, some of which will be discussed later. However, it will be shown that neutron irradia­tion can strongly affect both the microstructure and microchemistry of stainless steels and high-nickel alloys, with strong consequences on physical proper­ties, mechanical properties, dimensional stability, and structural integrity.

Stainless steels are currently being used or have been used as structural materials in a variety of nuclear environments, most particularly in sodium — cooled fast reactors, water-cooled and water-moderated test reactors, water-cooled and water-moderated power reactors, with the latter subdivided into light water and heavy water types. Additionally, there are reactor types involving the use of other coolants (helium, lithium, NaK, lead, lead-bismuth eutectic, mercury, molten salt, organic liquids, etc.) and other moderators such as graphite or beryllium.

The preceding reactor types are based on the fission of uranium and/or plutonium, producing neutron energy distributions peaking at ^2 MeV prior to moderation and leakage effects that produce the operating spectrum. However, there are more energetic sources of neutrons in fusion-derived spectra, with the source peaking at ^14 MeV and especially from spallation events occurring at ener­gies of hundreds of MeV, although most spallation spectra are mixtures of high-energy protons and neutrons. It is important to note that in each of these various reactors, there are not only significant differences in neutron flux-spectra but also signifi­cant differences in neutron fluence experienced by structural components. These differences in fluence arise not only from differences in neutron flux characteristic of the different reactor types but also the location of the steel relative to the core. For instance, boiling water reactors and pressurized water reactors have similar in-core spectra, but stainless steels in boiling water reactors are located much farther from the core, resulting in a factor of reduction of ^20 in both neutron dose rate and accumulated dose compared to steels in pressurized water reactors.